1. Field of the Invention
The present invention pertains to the delivery of a flow of breathing gas to the airway of a patient and more particularly to an apparatus and method for providing improved comfort for a patient receiving a flow of breathing gas.
2. Description of the Related Art
There are numerous situations where it is necessary or desirable to deliver a flow of breathing gas to the airway of a patient. For example, it is known to deliver a flow of breathing gas to a patient during at least a portion of the breathing cycle to treat breathing and/or cardiac disorders such as obstructive sleep apnea syndrome, chronic obstructive pulmonary disease, congestive heart failure, and other respiratory and/or breathing disorders.
While sleeping, a patient suffering from obstructive sleep apnea syndrome (OSAS) is prone to having their airway narrow and/or collapse due to, for instance, mechanical collapsing forces that result from the structure of the airway tissues, poor muscle tone, and body position. One method of treating OSAS is continuous positive airway pressure (CPAP) therapy. With CPAP therapy, a flow of breathing gas is supplied at a constant pressure of sufficient magnitude to splint the patient's airway open and to prevent the narrowing and/or collapse of the airway.
During a normal breathing cycle, however, the pressure gradient between the lungs and the exterior of the body is not constant. For example during inspiration, the pressure gradient (sometimes referred to as the “inspiratory pressure gradient”) falls from zero at the start of the inspiratory phase to a peak negative value and then rises back to zero at the end of the inspiratory phase. During expiration, the pressure gradient (sometimes referred to as the “expiratory pressure gradient”) rises from zero at the start of the expiration phase to a peak value and then falls back to zero at the end of the expiratory phase. Because the pressure gradient varies over the breathing cycle, ideally the pressure necessary to overcome airway collapse should vary accordingly over the breathing cycle. Thus, although CPAP provides a simple treatment solution for OSAS, the application of a constant splinting pressure to the airway exposes the patient to pressures that are higher than the pressures needed to support the airway for most of the breathing cycle.
Advanced therapies, such as bi-level positive airway pressure (bi-level PAP) therapies and proportional positive airway pressure therapies, seek to take advantage of the different pressure requirements and lower the pressure at certain instances during the breathing cycle. In bi-level PAP therapy, for example, a flow of breathing gas is supplied to a patient's airway at a first pressure during the inspiratory phase and a flow of breathing gas at a lower pressure is supplied to the patient's airway during the expiratory phase. These advanced therapies, however, may cause discomfort because the patient must still overcome the resistance created by the low pressure flow of breathing gas supplied during the expiratory phase.
Accordingly, a need exists for an apparatus and method for providing improved comfort for a patient receiving a flow of breathing gas which overcomes these and other problems associated with known systems.